This study will use a controlled, open-label, crossover design to characterize the pharmacokinetics of methadone during concomitant administration with Combivir in opiate-dependent HIV negative individuals. Subjects must be enrolled in a certified methadone maintenance program, during which the current methadone dose has remained unchanged for 30 days prior to enrollment into this study. Methadone-treated subjects will serve as their own control for the methadone pharmacokinetic analysis.